The invention relates to a method for delignification of chemical wood pulp by oxygen-alkali treatment.
Delignification with oxygen and alkali is today an industrially accepted process. The process is usually conducted as a pre-bleaching step before the final bleaching with chlorine containing chemicals. The kappa number of the pulp is then reduced usually from about 35 to 30 to a value of 20 to 15, implying a degree of delignification of about 40 to 50%. The values refer to oxygen-alkali delignification of sulphate pulps of coniferous woods.
It is also known that sulphate pulp having a kappa number within the range of 50 to 70 can be pre-bleached with oxygen and alkali. The reduction of the kappa number in such an oxygen-alkali delignification stage is usually restricted to 25 to 30 kappa number units.
Delignification with oxygen and alkali can be carried out both at high pulp consistency (25 to 30%) and at medium pulp consistency (7 to 10%). In the oxygen-alkali treatment of sulphate pulps having a kappa number in the range of 50 to 70 the process is usually carried out at medium pulp consistency. In the oxygen-alkali treatment of sulphate pulps having a kappa number in the range of 30 to 35 a high pulp consistency is mainly used.
In processes based on medium pulp consistency hydraulic reactors are used, i.e. liquid filled reactors having no gas phase in the reactor. The oxygen gas must be dispersed as small gas bubbles in the liquid phase surrounding the fibres. This means that there exists an upper limit for the amount of oxygen gas that can be charged to the reactor together with the pulp. This upper limit is defined by the pulp consistency, the reactor pressure and the reactor temperature. At a reactor pressure of 0,6 MPa, a pulp consistency of 8 to 10% and a reactor temperature of 95 to 110.degree. C. the oxygen charge is restricted to .about.40 kg O.sub.2 per ton of 100% unbleached pulp. The kappa number reduction in such a stage using a medium pulp consistency is restricted to about 30 kappa number units.
In a reactor system based on high pulp consistency there is always a gas phase of oxygen present in the reactor. The charge of alkali governs the kappa number reduction. The strength characteristics of the pulp usually represent a limit for the kappa number reduction or the degree of delignification. A high alkali charge leads to a high alkali concentration. Furthermore, there is a relation between the carbohydrate decomposition and the alkali concentration. At high alkali concentrations the carbohydrate decomposition measured as the intrinsic viscosity of the pulp or yield loss is strongly increased. In order to reduce the decomposition of the carbohydrates magnesium salts are usually added. Another method known from the literature is to treat the sulphate pulp with an acid solution having a pH value of &lt;4 prior to the oxygen-alkali delignification stage. In this manner heavy metal iones are removed from the pulp, whereby the decomposition of carbohydrates is reduced.
It is still necessary to limit the alkali charge to about 25 to 35 kg NaOH per ton of 100% unbleached pulp in order to limit the carbohydrate decomposition to an acceptable level. This limits the kappa number reduction to about 20 units.